1. Field of the Invention
The present invention relates to a sputtering apparatus and method for fabricating durable dielectric thin films such as those used for optical, semiconductor, tribological, appearance and other applications such as quarter wave stacks for laser mirrors.
2. Prior Art
Thin films are often fabricated by condensing vapors upon a collecting surface within a vacuum vessel. Vapors of the desirable thin film materials are produced by heating or providing energy in some other manner. Thin films made in this way are used to enhance the properties of the substrates upon which they are deposited, as in the case of anti-reflection coatings on optical lenses.
Thin films are also used for their own intrinsic properties apart from the substrate upon which they rest. Chrome coatings on automobile grills is an example of one such use. Thin beryllium windows for x-ray instrumentation are fabricated using thin film deposition. After deposition, the thin film is parted from the substrate and used alone.
The family of vapor phase processes of which the current method is a member are generally referred to as physical vapor deposition methods. Two methods that are used to convert the parent material into a vapor form are represented as evaporation and sputtering.
The evaporation process involves raising the temperature of the parent material by resistance, inductive or electron beam heating. The material for deposition vaporizes directly, or after melting. A major drawback of this method is that a compound material will usually chemically disassociate allowing the components with highest vapor pressure to vaporize first.
Chemical disassociation produces a thin film whose properties and chemical composition vary throughout the film thickness. For this reason, the evaporation method is primarily limited to the coating of single elements such as copper, aluminum and gold.
By using multiple sources, clustered together, it has been possible to condense more than one element onto a substrate simultaneously, thereby forming alloys. It is also possible to form alloys by holding several elemental components in the liquid state in a common bath, and by controlling the liquid inventory of each, to maintain a vapor cloud composed of these elemental components in a specified ratio.
The sputtering process involves accelerating a population of ions, extracted from a plasma, toward a parent material usually referred to as a target. Under preferred conditions, as the ions collide with the target surface, a significant number of surface atoms are ejected which form a vapor cloud. The pressure in the vessel is held at a level that results in few collisions between the accelerated ions and the ejected vapor atoms. The vapor moves away from the target surface with considerable kinetic energy. If the parent material is a chemical compound or alloy, atoms of each of the constituents are found in the vapor phase and condense upon the substrate. Generally, because all the atomic species necessary to reform the compound or alloy of the target are present in the condensate, the original compound or alloy is reformed in the thin film.
Sputtering is most commonly practiced by filling the vacuum vessel with an inert gas which is then ionized forming a low energy plasma. A negative electrical potential is applied to the target material which then attracts positive ions from the plasma which bombard the target surface causing the sputtering of atoms into the vapor phase. The vapor which is electrically neutral, condenses upon the nearby substrate forming the thin film.
An improved method uses a large area ion beam source to bombard the target surface causing a vapor flux to be generated by sputtering action. The gas pressure in the chamber, e.g. at the substrate surface, using this approach can be in the tenths or hundredths of a millitorr range. This is a great advantage since the finished film tends to contain fewer gas atoms and have an improved grain structure and atomic packing density. An extension of this method uses an elemental target to produce a vapor flux by ion beam bombardment as described above, plus the introduction of a chemically reactive gas into the chamber in order to form a thin film upon the substrate which is a chemical compound of the reaction between the target vapor atoms condensing on the substrate with the reactive gas atoms introduced separately.
This application uses the method described above but introduces a second ion beam source which is used to bombard the substrate directly wtth a gas mixture partly composed of the required reactive gas species. By controlling the parameters associated with this second ion beam, it is possible to improve the physical, electrical, crystal morphology and stoichiometry pooperties of the thin films which are formed. By. controlling certain parameters of the first ion beam and the target and substrate motion, it is possible to improve the uniformity, homogeneity and purity of the thin films.
This application also relates to U.S. Pat. No. 4,142,958, titled "Method for Fabricating Multilayer Optical Films", issued Mar. 6, 1979 for inventors David T. Wei and Anthony W. Louderback and assigned to Litton Systems Inc. of Woodland Hills, Calif. This application provides a novel improvement to the method of the Litton patent of Wei and Louderback.
The authors of the Wei and Louderback '958 patent provided a prior art discussion of quarter wave stacks. A part of that discussion is repeated here for the reader's convenience.
Quarter wave stacks and their design are explained in detail in the Military Standardization Handbook entitled, "Optical Design," MIL-HDBK141, Oct. 5, 1962. Each layer or thin film dielectric coating in a quarter wave stack has a thickness of about one quarter of a wavelength of the light which it is designed to reflect. The number of layers which comprise the quarter wave stack depends on the degree of desired reflectance and the differences in refractive indices of the layers. To increase reflectance, the number of layers and/or the differences in refractive indices may be increased. For mirrors used in ring lasers, the quarter wave stacks generally consist of 17 to 25 quarter wave thin film optical layers deposited on a substrate. Each layer is typically from 500 to 800 Angstroms thick. The layers alternate between a material of high index of refraction and a material of low index of refraction. Typically, the high index material is tantalum pentoxide (Ta.sub.2 O.sub.5) or titanium dioxide (TiO.sub.2) and the low index material is silicon dioxide (SiO.sub.2, i.e., quartz).
The principal method of fabricating quarter wave stacks for ring laser mirrors has been to use an electron beam evaporation technique. A substrate on which a reflective stack is to be coated is located inside of a vacuum chamber with a sample of the bulk or target material which is to be deposited. An electron beam focused on the sample material causes localized heating of the material to a point where molecules are evaporated off. These molecules then condense on the other surfaces located in the interior of the vacuum chamber, including the substrate which is being coated.
The process of electron beam evaporation as a means of coating is thoroughly explained in the text, "Physical Vapor Deposition," distributed by Airco Temescal, 2850 7th Street, Berkeley, Calif., 1976.
One problem encountered in using the electron beam evaporation technique is that molecules of the target material condense on the substrate in such a manner that voids are left between them. The resulting coating is less dense than the bulk target material, which results in a difference in the layer's index of refraction making it difficult to control the refractive indices of the stack.
The electron beam technique has the added disadvantage of producing localized high heat concentrations that result in small explosions which throw out larger chunks of the melt including impurities which condense in the layer.